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Sunday, August 17, 2014

Iraqi forces, aided by U.S. jets, claim swift gains in push to retake dam from militant

Iraqi forces, aided by U.S. jets, claim swift gains in push to retake dam from militant

The U.S. military released video of airstrikes on Islamic State targets near the Mosul Dam on Saturday. This video shows a U.S. airstrike on an Islamic State Humvee. (CENTCOM via YouTube)
August 17 at 8:29 PM

Islamic State fighters were on the run in northern Iraq on Sunday after Iraqi and Kurdish forces, backed by U.S. airstrikes, closed in on a strategically vital dam in the most significant attempt yet to reverse the militants’ blitz through Iraq.

Iraqi and Kurdish commanders claimed to be making swift progress, slicing through a series of villages and then reaching the dam after a wave of U.S. attacks in which fighter jets, drones and bombers pummeled the extremists’ positions.

It was the biggest offensive since the latest U.S. intervention in Iraq was announced 10 days ago, and it signaled an expansion of what was originally defined as a narrowly focused mission to protect American personnel in Iraq and help fleeing Yazidi villagers trapped on a mountain.
In a letter released Sunday notifying Congress of the action, President Obama said the militants’ control of the dam posed a threat to the U.S. Embassy 200 miles away in Baghdad, which could be inundated if the dam were breached.

“The failure of the Mosul Dam could threaten the lives of large numbers of civilians, endanger U.S. personnel and facilities, including the U.S. Embassy in Baghdad, and prevent the Iraqi government from providing critical services to the Iraqi populace,” he wrote.
 
As Yazidis try to flee Iraq, the Pentagon releases video that it says shows U.S. military airstrikes against Islamic State targets near the Mosul Dam. (Reuters)

Obama had signaled in a statement last week that protecting “critical infrastructure” would be part of what officials have described as a limited military intervention. This was, however, the first time Iraqi, Kurdish and U.S. forces had come together to launch a major ground assault.

A week ago, U.S. airstrikes helped clear Islamic State positions, enabling Kurdish fighters to retake two small towns south of the Kurdish capital, Irbil. That marked the Kurds’ first successful effort to recapture territory they had lost to an Islamic State offensive launched two weeks ago.

Kurdish and Iraqi officials said that Sunday’s operation was going better than expected and that the dam would soon be under full government control. “We expect to finish this within hours,” said Helgurd Hikmat, a spokesman for the Kurdish forces, known as the pesh merga.

A U.S. official, who spoke on the condition of anonymity because of the sensitivity of the situation, also said that the operation had “made significant progress.” But he said that recapturing the dam would take time “because there are a lot of IEDs,” or roadside bombs.

Late Sunday night, a senior Kurdish official said that Islamic State fighters had abandoned their positions at the dam but that Iraqi and Kurdish forces had refrained from entering the facility because of concerns that it was booby-trapped.

“Everybody is being really careful about their sinister tactics. When they leave their positions, they mine them,” said Hoshyar Zebari, a former Iraqi foreign minister who is working closely with the Kurdish government.

“But we don’t see any resistance whatsoever.”
 
The U.S. military released video of targeted airstrikes near the Mosul Dam in Iraq over the weekend. The strikes conducted Sunday damaged or destroyed several Islamic State assets, including armored vehicles and Humvees, according to U.S. Central Command. (CENTCOM via YouTube)
 
‘Beat them, beat them’

The Islamic State’s Aug. 7 capture of the Mosul Dam, just hours before Obama announced his decision to send U.S. warplanes back into action in Iraq, was a high point in the group’s campaign to establish a caliphate across the Middle East, putting the militants in control of one of Iraq’s most vital facilities.

Ten days on, it seemed that the intervention was starting to turn the tide.

At the Badriya checkpoint, six miles north of the dam, spirits were high among pesh merga troops blocking the road ahead, citing the danger posed by explosives planted by the retreating militants. Several Islamic State fighters had been captured trying to sneak through Kurdish checkpoints in a bid to escape, said Yunus Said, a volunteer fighter. Others had retreated to the western bank of the Tigris River, he said.

As he spoke, a convoy of SUVs and armored vehicles sped past from the direction of the front line, escorting a pickup in which a bound, blindfolded captive sat.

The soldiers cheered. “Daish,” they shouted, using the Arabic acronym for the Islamic State. “Beat them, beat them.”

Iraq’s elite special forces, which worked closely alongside U.S. Special Forces units before U.S. troops withdrew in 2011, took the lead in the fighting around the dam, while pesh merga troops closed in on the surrounding villages from the north. Brig. Gen. Abdulwahab al-Saidi, a commander with the Iraqi special forces, said the Iraqi air force and SWAT teams also were involved.

Their advance was preceded by the most intense U.S. bombardment yet, with 14 airstrikes destroying armed vehicles, Humvees, armored personnel carriers and a checkpoint belonging to the militants, according to U.S. Central Command statements. The strikes followed nine in the area the previous day. Three more were carried out later Sunday.

The assault was the worst setback for the Islamic State since the militants embarked on their stunning rout of the Iraqi army across northern Iraq in June. The group has since continued to expand across Iraq and Syria .

The extremists also came under pressure in Syria on Sunday, with activists in their northern stronghold of Raqqah reporting 23 bombing raids by Syrian government warplanes against Islamic State targets there. The Britain-based Syrian Observatory for Human Rights said there were 84 Syrian airstrikes Sunday, an unusually high number. Of them, 43 were against the Islamic State, signaling a significant escalation of Syrian attacks against the group, which the government had for many months steered away from confronting.
 
Fears of flooding

On Sunday, two U.S. officials said that the Obama administration had agreed to requests from the Iraqi government to help its forces retake control of the dam because of its strategic importance.

If breached, the dam would unleash catastrophic flooding across a vast swath of territory as far south as Baghdad. But Kurdish and U.S. officials said fears that the militants would blow it up have been overstated. Among other things, it would be difficult to assemble enough explosives to do so.

Moreover, said Brig. Gen. Azad Hawezi, a senior Kurdish commander, “they would flood themselves first, because the first place that would disappear would be Mosul,” the biggest city controlled by the Islamic State immediately south of the dam.

However, U.S. officials have said that the dam was poorly constructed and requires constant maintenance and upkeep — something Islamic State fighters would be unable to provide, heightening the risk of failure over the long term.

If the dam were to remain in the hands of the Islamic State, “it could have tremendous humanitarian impacts on the country,” said a senior U.S. defense official, speaking on the condition of anonymity to discuss ongoing military operations. “Having them in control of the dam is threat enough.”

In a statement, U.S. Central Command said the airstrikes Sunday were carried out by a mix of fighter jets, armed drones and bomber aircraft.

The statement did not identify the type of bombers involved, but the Air Force has B-1 bombers based in the Persian Gulf at al-Udeid Air Base in Qatar. It is thought to be the first time that bomber aircraft have been involved in the Iraqi air campaign. Fighter jets involved in the attacks have largely come from the USS George H.W. Bush, an aircraft carrier deployed to the gulf.

No U.S. troops or military advisers are embedded with Iraqi or Kurdish forces, according to American officials, although about 70 U.S. troops are based at a joint operations center in Irbil.

Whitlock reported from Washington, and Morris reported from Baghdad. Karen DeYoung in Washington contributed to this report.
 
Craig Whitlock covers the Pentagon and national security. He has reported for The Washington Post since 1998.
 
 
Loveday Morris is a Beirut-based correspondent for The Post. She has previously covered the Middle East for The National, based in Abu Dhabi, and for the Independent, based in London and Beirut.

World Wide Web

World Wide Web

From Wikipedia, the free encyclopedia
 
The World Wide Web (abbreviated as WWW or W3,[1] commonly known as the Web) is a system of interlinked hypertext documents that are accessed via the Internet. With a web browser, one can view web pages that may contain text, images, videos, and other multimedia and navigate between them via hyperlinks.
Tim Berners-Lee, a British computer scientist and former CERN employee,[2] is considered the inventor of the Web.[3] On March 12, 1989,[4] he wrote a proposal for what would eventually become the World Wide Web.[5] The 1989 proposal was meant for a more effective CERN communication system but Berners-Lee eventually realised the concept could be implemented throughout the world.[6] Berners-Lee and Belgian computer scientist Robert Cailliau proposed in 1990 to use hypertext "to link and access information of various kinds as a web of nodes in which the user can browse at will",[7] and Berners-Lee finished the first website in December of that year.[8] The first test was completed around 20 December 1990 and Berners-Lee reported about the project on the newsgroup alt.hypertext on 7 August 1991.[9]

History

The NeXT Computer used by Tim Berners-Lee at CERN.

In the May 1970 issue of Popular Science magazine, Arthur C. Clarke predicted that satellites would someday "bring the accumulated knowledge of the world to your fingertips" using a console that would combine the functionality of the photocopier, telephone, television and a small computer, allowing data transfer and video conferencing around the globe.[10]

On March 12, 1989, Tim Berners-Lee wrote a proposal that referenced ENQUIRE, a database and software project he had built in 1980, and described a more elaborate information management system.[11]

With help from Robert Cailliau, he published a more formal proposal (on 12 November 1990) to build a "Hypertext project" called "WorldWideWeb" (one word, also "W3") as a "web" of "hypertext documents" to be viewed by "browsers" using a client–server architecture.[7] This proposal estimated that a read-only web would be developed within three months and that it would take six months to achieve "the creation of new links and new material by readers, [so that] authorship becomes universal" as well as "the automatic notification of a reader when new material of interest to him/her has become available." While the read-only goal was met, accessible authorship of web content took longer to mature, with the wiki concept, WebDAV, blogs, Web 2.0 and RSS/Atom.[12]

The proposal was modeled after the SGML reader Dynatext by Electronic Book Technology, a spin-off from the Institute for Research in Information and Scholarship at Brown University. The Dynatext system, licensed by CERN, was a key player in the extension of SGML ISO 8879:1986 to Hypermedia within HyTime, but it was considered too expensive and had an inappropriate licensing policy for use in the general high energy physics community, namely a fee for each document and each document alteration.
The CERN datacenter in 2010 housing some WWW servers

A NeXT Computer was used by Berners-Lee as the world's first web server and also to write the first web browser, WorldWideWeb, in 1990. By Christmas 1990, Berners-Lee had built all the tools necessary for a working Web:[13] the first web browser (which was a web editor as well); the first web server; and the first web pages,[14] which described the project itself.

The first web page may be lost, but Paul Jones of UNC-Chapel Hill in North Carolina announced in May 2013 that Berners-Lee gave him what he says is the oldest known web page during a 1991 visit to UNC. Jones stored it on a magneto-optical drive and on his NeXT computer.[15]

On 6 August 1991, Berners-Lee published a short summary of the World Wide Web project on the newsgroup alt.hypertext.[16] This date also marked the debut of the Web as a publicly available service on the Internet, although new users only access it after August 23. For this reason this is considered the internaut's day. Several newsmedia have reported that the first photo on the Web was published by Berners-Lee in 1992, an image of the CERN house band Les Horribles Cernettes taken by Silvano de Gennaro; Gennaro has disclaimed this story, writing that media were "totally distorting our words for the sake of cheap sensationalism."[17]

The first server outside Europe was installed at the Stanford Linear Accelerator Center (SLAC) in Palo Alto, California, to host the SPIRES-HEP database. Accounts differ substantially as to the date of this event. The World Wide Web Consortium says December 1992,[18] whereas SLAC itself claims 1991.[19][20] This is supported by a W3C document titled A Little History of the World Wide Web.[21]

The underlying concept of hypertext originated in previous projects from the 1960s, such as the Hypertext Editing System (HES) at Brown University, Ted Nelson's Project Xanadu, and Douglas Engelbart's oN-Line System (NLS). Both Nelson and Engelbart were in turn inspired by Vannevar Bush's microfilm-based memex, which was described in the 1945 essay "As We May Think".[22]

Berners-Lee's breakthrough was to marry hypertext to the Internet. In his book Weaving The Web, he explains that he had repeatedly suggested that a marriage between the two technologies was possible to members of both technical communities, but when no one took up his invitation, he finally assumed the project himself. In the process, he developed three essential technologies:
The World Wide Web had a number of differences from other hypertext systems available at the time. The Web required only unidirectional links rather than bidirectional ones, making it possible for someone to link to another resource without action by the owner of that resource. It also significantly reduced the difficulty of implementing web servers and browsers (in comparison to earlier systems), but in turn presented the chronic problem of link rot. Unlike predecessors such as HyperCard, the World Wide Web was non-proprietary, making it possible to develop servers and clients independently and to add extensions without licensing restrictions. On 30 April 1993, CERN announced that the World Wide Web would be free to anyone, with no fees due.[24] Coming two months after the announcement that the server implementation of the Gopher protocol was no longer free to use, this produced a rapid shift away from Gopher and towards the Web. An early popular web browser was ViolaWWW for Unix and the X Windowing System.
Robert Cailliau, Jean-François Abramatic of IBM, and Tim Berners-Lee at the 10th anniversary of the World Wide Web Consortium.

Scholars generally agree that a turning point for the World Wide Web began with the introduction[25] of the Mosaic web browser[26] in 1993, a graphical browser developed by a team at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign (NCSA-UIUC), led by Marc Andreessen. Funding for Mosaic came from the U.S. High-Performance Computing and Communications Initiative and the High Performance Computing and Communication Act of 1991, one of several computing developments initiated by U.S. Senator Al Gore.[27] Prior to the release of Mosaic, graphics were not commonly mixed with text in web pages and the web's popularity was less than older protocols in use over the Internet, such as Gopher and Wide Area Information Servers (WAIS). Mosaic's graphical user interface allowed the Web to become, by far, the most popular Internet protocol.

The World Wide Web Consortium (W3C) was founded by Tim Berners-Lee after he left the European Organization for Nuclear Research (CERN) in October 1994. It was founded at the Massachusetts Institute of Technology Laboratory for Computer Science (MIT/LCS) with support from the Defense Advanced Research Projects Agency (DARPA), which had pioneered the Internet; a year later, a second site was founded at INRIA (a French national computer research lab) with support from the European Commission DG InfSo; and in 1996, a third continental site was created in Japan at Keio University. By the end of 1994, the total number of websites was still relatively small, but many notable websites were already active that foreshadowed or inspired today's most popular services.

Connected by the existing Internet, other websites were created around the world, adding international standards for domain names and HTML. Since then, Berners-Lee has played an active role in guiding the development of web standards (such as the markup languages to compose web pages in), and has advocated his vision of a Semantic Web. The World Wide Web enabled the spread of information over the Internet through an easy-to-use and flexible format. It thus played an important role in popularizing use of the Internet.[28] Although the two terms are sometimes conflated in popular use, World Wide Web is not synonymous with Internet.[29] The Web is a collection of documents and both client and server software using Internet protocols such as TCP/IP and HTTP.

Tim Berners-Lee was knighted in 2004 by Queen Elizabeth II for his contribution to the World Wide Web.[30]

Function

The terms Internet and World Wide Web are often used in everyday speech without much distinction. However, the Internet and the World Wide Web are not the same. The Internet is a global system of interconnected computer networks. In contrast, the web is one of the services that runs on the Internet. It is a collection of text documents and other resources, linked by hyperlinks and URLs, usually accessed by web browsers from web servers.[31]

Viewing a web page on the World Wide Web normally begins either by typing the URL of the page into a web browser, or by following a hyperlink to that page or resource. The web browser then initiates a series of background communication messages to fetch and display the requested page. In the 1990s, using a browser to view web pages—and to move from one web page to another through hyperlinks—came to be known as 'browsing,' 'web surfing,' (after channel surfing), or 'navigating the Web'. Early studies of this new behavior investigated user patterns in using web browsers. One study, for example, found five user patterns: exploratory surfing, window surfing, evolved surfing, bounded navigation and targeted navigation.[32]

The following example demonstrates the functioning of web browser when accessing a page at the URL http://example.org/wiki/World_Wide_Web. The browser resolves the server name of the URL (example.org) into an Internet Protocol address using the globally distributed Domain Name System (DNS). This lookup returns an IP address such as 208.80.152.2. The browser then requests the resource by sending an HTTP request across the Internet to the computer at that address. It requests service from a specific TCP port number that is well known for the HTTP service, so that the receiving host can distinguish an HTTP request from other network protocols it may be servicing. The HTTP protocol normally uses port number 80. The content of the HTTP request can be as simple as two lines of text:
 
GET /wiki/World_Wide_Web HTTP/1.1
Host: example.org

The computer receiving the HTTP request delivers it to web server software listening for requests on port 80. If the web server can fulfill the request it sends an HTTP response back to the browser indicating success:
 
HTTP/1.0 200 OK
Content-Type: text/html; charset=UTF-8

followed by the content of the requested page. The Hypertext Markup Language for a basic web page looks like Example.org – The World Wide Web The World Wide Web, abbreviated as WWW and commonly known ...


The web browser parses the HTML and interprets the markup (</tt>, <tt><p> </tt> for paragraph, and such) that surrounds the words to format the text on the screen. Many web pages use HTML to reference the URLs of other resources such as images, other embedded media, <a href="http://en.wikipedia.org/wiki/Client-side_scripting" title="Client-side scripting">scripts</a> that affect page behavior, and <a href="http://en.wikipedia.org/wiki/Cascading_Style_Sheets" title="Cascading Style Sheets">Cascading Style Sheets</a> that affect page layout. The browser makes additional HTTP requests to the web server for these other <a href="http://en.wikipedia.org/wiki/Internet_media_type" title="Internet media type">Internet media types</a>. As it receives their content from the web server, the browser progressively <a href="http://en.wikipedia.org/wiki/Layout_engine" title="Layout engine" class="mw-redirect">renders</a> the page onto the screen as specified by its HTML and these additional resources.</p> <h3> <span class="mw-headline" id="Linking">Linking</span></h3> <p> Most web pages contain <a href="http://en.wikipedia.org/wiki/Hyperlinks" title="Hyperlinks" class="mw-redirect">hyperlinks</a> to other related pages and perhaps to downloadable files, source documents, definitions and other web resources. In the underlying HTML, a hyperlink looks like <a href="<i>http://example.org/wiki/Main_Page</i>"><i>Example.org, a free encyclopedia</i></a></p> <div class="thumb tright"> <div class="thumbinner" style="width:222px;"> <a href="http://en.wikipedia.org/wiki/File:WorldWideWebAroundWikipedia.png" class="image"><img alt="" src="http://upload.wikimedia.org/wikipedia/commons/thumb/b/b9/WorldWideWebAroundWikipedia.png/220px-WorldWideWebAroundWikipedia.png" class="thumbimage" data-file-width="1185" data-file-height="853" height="158" width="220"></a> <div class="thumbcaption"> <div class="magnify"> <a href="http://en.wikipedia.org/wiki/File:WorldWideWebAroundWikipedia.png" class="internal" title="Enlarge"><img src="http://bits.wikimedia.org/static-1.24wmf16/skins/common/images/magnify-clip.png" alt="" height="11" width="15"></a></div> Graphic representation of a minute fraction of the WWW, demonstrating <a href="http://en.wikipedia.org/wiki/Hyperlink" title="Hyperlink">hyperlinks</a></div> </div> </div> <p>  </p> <p> Such a collection of useful, related resources, interconnected via hypertext links is dubbed a <i>web</i> of information. Publication on the Internet created what <a href="http://en.wikipedia.org/wiki/Tim_Berners-Lee" title="Tim Berners-Lee">Tim Berners-Lee</a> first called the <i>WorldWideWeb</i> (in its original <a href="http://en.wikipedia.org/wiki/CamelCase" title="CamelCase">CamelCase</a>, which was subsequently discarded) in November 1990.<sup id="cite_ref-W90_7-2" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-W90-7"><span>[</span>7<span>]</span></a></sup></p> <p> The hyperlink structure of the WWW is described by the <a href="http://en.wikipedia.org/wiki/Webgraph" title="Webgraph">webgraph</a>: the nodes of the <a href="http://en.wikipedia.org/wiki/Webgraph" title="Webgraph">webgraph</a> correspond to the web pages (or URLs) the directed edges between them to the <a href="http://en.wikipedia.org/wiki/Hyperlink" title="Hyperlink">hyperlinks</a>.</p> <p>  </p> <p> Over time, many web resources pointed to by hyperlinks disappear, relocate, or are replaced with different content. This makes hyperlinks obsolete, a phenomenon referred to in some circles as <a href="http://en.wikipedia.org/wiki/Link_rot" title="Link rot">link rot</a> and the hyperlinks affected by it are often called <a href="http://en.wikipedia.org/wiki/Dead_link" title="Dead link" class="mw-redirect">dead links</a>. The ephemeral nature of the Web has prompted many efforts to archive web sites. The <a href="http://en.wikipedia.org/wiki/Internet_Archive" title="Internet Archive">Internet Archive</a>, active since 1996, is the best known of such efforts.</p> <h3> <span class="mw-headline" id="Dynamic_updates_of_web_pages">Dynamic updates of web pages</span></h3> <a href="http://en.wikipedia.org/wiki/JavaScript" title="JavaScript">JavaScript</a> is a <a href="http://en.wikipedia.org/wiki/Scripting_programming_language" title="Scripting programming language" class="mw-redirect">scripting language</a> that was initially developed in 1995 by <a href="http://en.wikipedia.org/wiki/Brendan_Eich" title="Brendan Eich">Brendan Eich</a>, then of <a href="http://en.wikipedia.org/wiki/Netscape" title="Netscape">Netscape</a>, for use within web pages.<sup id="cite_ref-Hamilton_33-0" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-Hamilton-33"><span>[</span>33<span>]</span></a></sup> The standardised version is <a href="http://en.wikipedia.org/wiki/ECMAScript" title="ECMAScript">ECMAScript</a>.<sup id="cite_ref-Hamilton_33-1" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-Hamilton-33"><span>[</span>33<span>]</span></a></sup> To make web pages more interactive, some web applications also use JavaScript techniques such as <a href="http://en.wikipedia.org/wiki/Ajax_%28programming%29" title="Ajax (programming)">Ajax</a> (<a href="http://en.wikipedia.org/wiki/Asynchronous_I/O" title="Asynchronous I/O">asynchronous</a> JavaScript and <a href="http://en.wikipedia.org/wiki/XML" title="XML">XML</a>). <a href="http://en.wikipedia.org/wiki/Client-side_scripting" title="Client-side scripting">Client-side script</a> is delivered with the page that can make additional HTTP requests to the server, either in response to user actions such as mouse movements or clicks, or based on lapsed time. The server's responses are used to modify the current page rather than creating a new page with each response, so the server needs only to provide limited, incremental information. Multiple Ajax requests can be handled at the same time, and users can interact with the page while data is retrieved. Web pages may also regularly <a href="http://en.wikipedia.org/wiki/Polling_%28computer_science%29" title="Polling (computer science)">poll</a> the server to check whether new information is available.<sup id="cite_ref-34" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-34"><span>[</span>34<span>]</span></a></sup> <h3> <span class="mw-headline" id="WWW_prefix">WWW prefix</span></h3> <p> Many hostnames used for the World Wide Web begin with <i>www</i> because of the long-standing practice of naming Internet hosts according to the services they provide. The <a href="http://en.wikipedia.org/wiki/Hostname" title="Hostname">hostname</a> of a <a href="http://en.wikipedia.org/wiki/Web_server" title="Web server">web server</a> is often <i>www</i>, in the same way that it may be <i>ftp</i> for an <a href="http://en.wikipedia.org/wiki/FTP_server" title="FTP server" class="mw-redirect">FTP server</a>, and <i>news</i> or <i>nntp</i> for a <a href="http://en.wikipedia.org/wiki/USENET" title="USENET" class="mw-redirect">USENET</a> <a href="http://en.wikipedia.org/wiki/News_server" title="News server">news server</a>. These host names appear as <a href="http://en.wikipedia.org/wiki/Domain_Name_System" title="Domain Name System">Domain Name System</a> or (DNS) <a href="http://en.wikipedia.org/wiki/Subdomain" title="Subdomain">subdomain</a> names, as in <i>www.example.com</i>. The use of <i>www</i> is not required by any technical or policy standard and many web sites do not use it; indeed, the first ever web server was called <i>nxoc01.cern.ch</i>.<sup id="cite_ref-35" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-35"><span>[</span>35<span>]</span></a></sup> </p> <p> According to Paolo Palazzi,<sup id="cite_ref-36" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-36"><span>[</span>36<span>]</span></a></sup> who worked at CERN along with Tim Berners-Lee, the popular use of <i>www</i> as subdomain was accidental; the World Wide Web project page was intended to be published at www.cern.ch while info.cern.ch was intended to be the CERN home page, however the DNS records were never switched, and the practice of prepending <i>www</i> to an institution's website domain name was subsequently copied. Many established websites still use the prefix, or they employ other subdomain names such as <i>www2</i>, <i>secure</i>, for special purposes. Many such web servers are set up so that both the main domain name (e.g., example.com) and the <i>www</i> subdomain (e.g., www.example.com) refer to the same site; others require one form or the other, or they may map to different web sites.</p> <p>  </p> <p> The use of a subdomain name is useful for <a href="http://en.wikipedia.org/wiki/Load_balancing_%28computing%29" title="Load balancing (computing)">load balancing</a> incoming web traffic by creating a <a href="http://en.wikipedia.org/wiki/CNAME_record" title="CNAME record">CNAME record</a> that points to a cluster of web servers. Since, currently, only a subdomain can be used in a CNAME, the same result cannot be achieved by using the bare domain root.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2013)">citation needed</span></a></i>]</sup></p> <p>  </p> <p> When a user submits an incomplete domain name to a web browser in its address bar input field, some web browsers automatically try adding the prefix "www" to the beginning of it and possibly ".com", ".org" and ".net" at the end, depending on what might be missing. For example, entering 'microsoft' may be transformed to <i>http://www.microsoft.com/</i> and 'openoffice' to <i>http://www.openoffice.org</i>. This feature started appearing in early versions of Mozilla <a href="http://en.wikipedia.org/wiki/Firefox" title="Firefox">Firefox</a>, when it still had the working title 'Firebird' in early 2003, from an earlier practice in browsers such as <a href="http://en.wikipedia.org/wiki/Lynx_%28web_browser%29" title="Lynx (web browser)">Lynx</a>.<sup id="cite_ref-37" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-37"><span>[</span>37<span>]</span></a></sup> It is reported that Microsoft was granted a US patent for the same idea in 2008, but only for mobile devices.<sup id="cite_ref-38" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-38"><span>[</span>38<span>]</span></a></sup></p> <p>  </p> <p> In English, <a href="http://en.wikipedia.org/wiki/Pronunciation_of_%22www%22" title="Pronunciation of "www""><i>www</i> is usually read as</a> <i>double-u double-u double-u</i>.<sup id="cite_ref-39" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-39"><span>[</span>39<span>]</span></a></sup> Some users pronounce it <i>dub-dub-dub</i>, particularly in New Zealand. Stephen Fry, in his "Podgrammes" series of podcasts, pronounces it <i>wuh wuh wuh</i>.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2013)">citation needed</span></a></i>]</sup> The English writer <a href="http://en.wikipedia.org/wiki/Douglas_Adams" title="Douglas Adams">Douglas Adams</a> once quipped in <a href="http://en.wikipedia.org/wiki/The_Independent" title="The Independent">The Independent on Sunday</a> (1999): "The World Wide Web is the only thing I know of whose shortened form takes three times longer to say than what it's short for".<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (September 2013)">citation needed</span></a></i>]</sup> In Mandarin Chinese, <i>World Wide Web</i> is commonly translated via a <a href="http://en.wikipedia.org/wiki/Phono-semantic_matching" title="Phono-semantic matching">phono-semantic matching</a> to <i>wàn wéi wǎng</i> (<span lang="zh"><a href="http://en.wiktionary.org/wiki/%E4%B8%87%E7%BB%B4%E7%BD%91" class="extiw" title="wikt:万维网">万维网</a></span>), which satisfies <i>www</i> and literally means "myriad dimensional net",<sup id="cite_ref-40" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-40"><span>[</span>40<span>]</span></a></sup> a translation that very appropriately reflects the design concept and proliferation of the World Wide Web. Tim Berners-Lee's web-space states that <i>World Wide Web</i> is officially spelled as three separate words, each capitalised, with no intervening hyphens.<sup id="cite_ref-41" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-41"><span>[</span>41<span>]</span></a></sup></p> <p>  </p> <p> Use of the www prefix is declining as <a href="http://en.wikipedia.org/wiki/Web_2.0" title="Web 2.0">Web 2.0</a> <a href="http://en.wikipedia.org/wiki/Web_application" title="Web application">web applications</a> seek to brand their domain names and make them easily pronounceable.<sup id="cite_ref-42" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-42"><span>[</span>42<span>]</span></a></sup> As the <a href="http://en.wikipedia.org/wiki/Mobile_web" title="Mobile web" class="mw-redirect">mobile web</a> grows in popularity, services like <a href="http://en.wikipedia.org/wiki/Google_Mail" title="Google Mail" class="mw-redirect">Gmail</a>.com, <a href="http://en.wikipedia.org/wiki/MySpace" title="MySpace" class="mw-redirect">MySpace</a>.com, <a href="http://en.wikipedia.org/wiki/Facebook" title="Facebook">Facebook</a>.com and <a href="http://en.wikipedia.org/wiki/Twitter" title="Twitter">Twitter</a>.com are most often discussed without adding www to the domain (or, indeed, the .com).</p> <h3> <span class="mw-headline" id="Scheme_specifiers">Scheme specifiers</span></h3> <p> The scheme specifiers <i>http://</i> and <i>https://</i> at the start of a web <a href="http://en.wikipedia.org/wiki/Uniform_Resource_Identifier" title="Uniform Resource Identifier" class="mw-redirect">URI</a> refer to <a href="http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol" title="Hypertext Transfer Protocol">Hypertext Transfer Protocol</a> or <a href="http://en.wikipedia.org/wiki/HTTP_Secure" title="HTTP Secure">HTTP Secure</a>, respectively. They specify the communication protocol to use for the request and response. The HTTP protocol is fundamental to the operation of the World Wide Web, and the added encryption layer in HTTPS is essential when browsers send or retrieve confidential data, such as passwords or banking information. Web browsers usually prepend http:// to user-entered URIs, if omitted.</p> <h2> <span class="mw-headline" id="Web_servers">Web servers</span></h2> The primary function of a web server is to deliver web pages in response to client requests. This means delivery of HTML documents and any additional content that may be included by a document, such as images, style sheets and scripts. <h2> <span class="mw-headline" id="Privacy">Privacy</span></h2> Every time a web server requests a web page, the server can identify the request's IP address and usually logs it. Also, unless set not to do so, most web browsers record requested web pages in a viewable <i>history</i> feature, and usually <a href="http://en.wikipedia.org/wiki/Web_cache" title="Web cache">cache</a> much of the content locally. Unless the server-browser communication uses HTTPS encryption, web requests and responses travel in plain text across the internet and can be viewed, recorded, and cached by intermediate systems. <p>  </p> <p> When a web page asks for, and the user supplies, <a href="http://en.wikipedia.org/wiki/Personally_identifiable_information" title="Personally identifiable information">personally identifiable information</a>—such as their real name, address, e-mail address, etc.—web-based entities can associate current web traffic with that individual. If the website uses <a href="http://en.wikipedia.org/wiki/HTTP_cookie" title="HTTP cookie">HTTP cookies</a>, username and password authentication, or other tracking techniques, it can relate other web visits, before and after, to the identifiable information provided. In this way it is possible for a web-based organisation to develop and build a profile of the individual people who use its site or sites. It may be able to build a record for an individual that includes information about their leisure activities, their shopping interests, their profession, and other aspects of their <a href="http://en.wikipedia.org/wiki/Demographic_profile" title="Demographic profile">demographic profile</a>. These profiles are obviously of potential interest to marketeers, advertisers and others. Depending on the website's <a href="http://en.wikipedia.org/wiki/Terms_and_conditions" title="Terms and conditions" class="mw-redirect">terms and conditions</a> and the local laws that apply information from these profiles may be sold, shared, or passed to other organisations without the user being informed. For many ordinary people, this means little more than some unexpected e-mails in their in-box, or some uncannily relevant advertising on a future web page. For others, it can mean that time spent indulging an unusual interest can result in a deluge of further targeted marketing that may be unwelcome. Law enforcement, counter terrorism and espionage agencies can also identify, target and track individuals based on their interests or proclivities on the Web.</p> <p>  </p> <p> <a href="http://en.wikipedia.org/wiki/Social_networking" title="Social networking" class="mw-redirect">Social networking</a> sites try to get users to use their real names, interests, and locations. They believe this makes the social networking experience more realistic, and therefore more engaging for all their users. On the other hand, uploaded photographs unguarded statements can be identified to an individual, who may regret this exposure. Employers, schools, parents, and other relatives may be influenced by aspects of social networking profiles that the posting individual did not intend for these audiences. <a href="http://en.wikipedia.org/wiki/Cyberbullying" title="Cyberbullying">On-line bullies</a> may make use of personal information to harass or <a href="http://en.wikipedia.org/wiki/Cyberstalking" title="Cyberstalking">stalk</a> users. Modern social networking websites allow fine grained control of the privacy settings for each individual posting, but these can be complex and not easy to find or use, especially for beginners.<sup id="cite_ref-43" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-43"><span>[</span>43<span>]</span></a></sup></p> <p>  </p> <p> Photographs and videos posted onto websites have caused particular problems, as they can add a person's face to an on-line profile. With modern and potential <a href="http://en.wikipedia.org/wiki/Facial_recognition_system" title="Facial recognition system">facial recognition technology</a>, it may then be possible to relate that face with other, previously anonymous, images, events and scenarios that have been imaged elsewhere. Because of image caching, mirroring and copying, it is difficult to remove an image from the World Wide Web.</p> <h2> <span class="mw-headline" id="Intellectual_property">Intellectual property</span></h2> The intellectual property rights for any creative work initially rest with its creator. Web users who want to publish their work onto the World Wide Web, however, must be aware of the details of the way they do it. If artwork, photographs, writings, poems, or technical innovations are published by their creator onto a privately owned web server, then they may choose the <a href="http://en.wikipedia.org/wiki/Copyright" title="Copyright">copyright</a> and other conditions freely themselves. This is unusual though; more commonly work is uploaded to websites and servers that are owned by other organizations. It depends upon the terms and conditions of the site or service provider to what extent the original owner automatically signs over rights to their work by the choice of destination and by the act of uploading.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (January 2013)">citation needed</span></a></i>]</sup> <p>  </p> <p> Some web users erroneously assume that anything they find online is freely available, as if it were in the <a href="http://en.wikipedia.org/wiki/Public_domain" title="Public domain">public domain</a>, which is not always the case. Content owners aware of this belief may expect that others will use their published content without permission. Therefore, some content publishers embed <a href="http://en.wikipedia.org/wiki/Digital_watermarking" title="Digital watermarking">digital watermarks</a> in media files, sometimes charging users to receive unmarked copies for legitimate use. <a href="http://en.wikipedia.org/wiki/Digital_rights_management" title="Digital rights management">Digital rights management</a> includes forms of access control technology that further limit the use of digital content even after it has been bought or downloaded.<sup class="noprint Inline-Template Template-Fact" style="white-space:nowrap;">[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:Citation_needed" title="Wikipedia:Citation needed"><span title="This claim needs references to reliable sources. (January 2013)">citation needed</span></a></i>]</sup></p> <h2> <span class="mw-headline" id="Security">Security</span></h2> <p> For criminals, the Web has become the preferred way to spread <a href="http://en.wikipedia.org/wiki/Malware" title="Malware">malware</a>. Cybercrime on the Web can include <a href="http://en.wikipedia.org/wiki/Identity_theft" title="Identity theft">identity theft</a>, fraud, espionage and <a href="http://en.wikipedia.org/wiki/Intelligence_gathering" title="Intelligence gathering" class="mw-redirect">intelligence gathering</a>.<sup id="cite_ref-Ben-Itzhak_44-0" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-Ben-Itzhak-44"><span>[</span>44<span>]</span></a></sup> Web-based <a href="http://en.wikipedia.org/wiki/Vulnerability_%28computing%29" title="Vulnerability (computing)">vulnerabilities</a> now outnumber traditional computer security concerns,<sup id="cite_ref-45" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-45"><span>[</span>45<span>]</span></a></sup><sup id="cite_ref-46" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-46"><span>[</span>46<span>]</span></a></sup> and as measured by <a href="http://en.wikipedia.org/wiki/Google" title="Google">Google</a>, about one in ten web pages may contain malicious code.<sup id="cite_ref-47" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-47"><span>[</span>47<span>]</span></a></sup> Most web-based <a href="http://en.wikipedia.org/wiki/Attack_%28computing%29" title="Attack (computing)">attacks</a> take place on legitimate websites, and most, as measured by <a href="http://en.wikipedia.org/wiki/Sophos" title="Sophos">Sophos</a>, are hosted in the United States, China and Russia.<sup id="cite_ref-Sophos-Q1-2008_48-0" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-Sophos-Q1-2008-48"><span>[</span>48<span>]</span></a></sup> The most common of all malware <a href="http://en.wikipedia.org/wiki/Threat_%28computer%29" title="Threat (computer)">threats</a> is <a href="http://en.wikipedia.org/wiki/SQL_injection" title="SQL injection">SQL injection</a> attacks against websites.<sup id="cite_ref-49" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-49"><span>[</span>49<span>]</span></a></sup> Through HTML and URIs, the Web was vulnerable to attacks like <a href="http://en.wikipedia.org/wiki/Cross-site_scripting" title="Cross-site scripting">cross-site scripting</a> (XSS) that came with the introduction of JavaScript<sup id="cite_ref-FGHR_50-0" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-FGHR-50"><span>[</span>50<span>]</span></a></sup> and were exacerbated to some degree by Web 2.0 and Ajax <a href="http://en.wikipedia.org/wiki/Web_design" title="Web design">web design</a> that favors the use of scripts.<sup id="cite_ref-51" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-51"><span>[</span>51<span>]</span></a></sup> Today by one estimate, 70% of all websites are open to XSS attacks on their users.<sup id="cite_ref-52" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-52"><span>[</span>52<span>]</span></a></sup></p> <p>  </p> <p> Proposed solutions vary to extremes. Large security vendors like <a href="http://en.wikipedia.org/wiki/McAfee" title="McAfee">McAfee</a> already design governance and compliance suites to meet post-9/11 regulations,<sup id="cite_ref-53" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-53"><span>[</span>53<span>]</span></a></sup> and some, like <a href="http://en.wikipedia.org/wiki/Finjan" title="Finjan">Finjan</a> have recommended active real-time inspection of code and all content regardless of its source.<sup id="cite_ref-Ben-Itzhak_44-1" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-Ben-Itzhak-44"><span>[</span>44<span>]</span></a></sup> Some have argued that for enterprise to see security as a business opportunity rather than a cost center,<sup id="cite_ref-54" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-54"><span>[</span>54<span>]</span></a></sup> "ubiquitous, always-on digital rights management" enforced in the infrastructure by a handful of organizations must replace the hundreds of companies that today secure data and networks.<sup id="cite_ref-55" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-55"><span>[</span>55<span>]</span></a></sup> <a href="http://en.wikipedia.org/wiki/Jonathan_Zittrain" title="Jonathan Zittrain">Jonathan Zittrain</a> has said users sharing responsibility for computing safety is far preferable to locking down the Internet.<sup id="cite_ref-56" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-56"><span>[</span>56<span>]</span></a></sup></p> <h2> <span class="mw-headline" id="Standards">Standards</span></h2> Many formal standards and other technical specifications and software define the operation of different aspects of the World Wide Web, the Internet, and computer information exchange. Many of the documents are the work of the World Wide Web Consortium (W3C), headed by Berners-Lee, but some are produced by the <a href="http://en.wikipedia.org/wiki/Internet_Engineering_Task_Force" title="Internet Engineering Task Force">Internet Engineering Task Force</a> (IETF) and other organizations. <p>  </p> <p> Usually, when web standards are discussed, the following publications are seen as foundational:</p> <ul> <li>Recommendations for <a href="http://en.wikipedia.org/wiki/Markup_languages" title="Markup languages" class="mw-redirect">markup languages</a>, especially <a href="http://en.wikipedia.org/wiki/HTML" title="HTML">HTML</a> and <a href="http://en.wikipedia.org/wiki/XHTML" title="XHTML">XHTML</a>, from the W3C. These define the structure and interpretation of <a href="http://en.wikipedia.org/wiki/Hypertext" title="Hypertext">hypertext</a> documents.</li> <li>Recommendations for <a href="http://en.wikipedia.org/wiki/Style_sheet_%28web_development%29" title="Style sheet (web development)">stylesheets</a>, especially <a href="http://en.wikipedia.org/wiki/Cascading_Style_Sheets" title="Cascading Style Sheets">CSS</a>, from the W3C.</li> <li>Standards for <a href="http://en.wikipedia.org/wiki/ECMAScript" title="ECMAScript">ECMAScript</a> (usually in the form of <a href="http://en.wikipedia.org/wiki/JavaScript" title="JavaScript">JavaScript</a>), from <a href="http://en.wikipedia.org/wiki/Ecma_International" title="Ecma International">Ecma International</a>.</li> <li>Recommendations for the <a href="http://en.wikipedia.org/wiki/Document_Object_Model" title="Document Object Model">Document Object Model</a>, from W3C.</li> </ul> <p> Additional publications provide definitions of other essential technologies for the World Wide Web, including, but not limited to, the following:</p> <ul> <li><i>Uniform Resource Identifier</i> (<a href="http://en.wikipedia.org/wiki/Uniform_Resource_Identifier" title="Uniform Resource Identifier" class="mw-redirect">URI</a>), which is a universal system for referencing resources on the Internet, such as hypertext documents and images. URIs, often called URLs, are defined by the IETF's <a class="external mw-magiclink-rfc" rel="nofollow" href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> / STD 66: <i>Uniform Resource Identifier (URI): Generic Syntax</i>, as well as its predecessors and numerous <a href="http://en.wikipedia.org/wiki/URI_scheme" title="URI scheme">URI scheme</a>-defining <a href="http://en.wikipedia.org/wiki/Request_For_Comments" title="Request For Comments" class="mw-redirect">RFCs</a>;</li> <li><i>HyperText Transfer Protocol (HTTP)</i>, especially as defined by <a class="external mw-magiclink-rfc" rel="nofollow" href="http://tools.ietf.org/html/rfc2616">RFC 2616</a>: <i>HTTP/1.1</i> and <a class="external mw-magiclink-rfc" rel="nofollow" href="http://tools.ietf.org/html/rfc2617">RFC 2617</a>: <i>HTTP Authentication</i>, which specify how the browser and server authenticate each other.</li> </ul> <h2> <span class="mw-headline" id="Accessibility">Accessibility</span></h2> There are methods for accessing the Web in alternative mediums and formats to facilitate use by individuals with disabilities. These disabilities may be visual, auditory, physical, speech related, cognitive, neurological, or some combination. Accessibility features also help people with temporary disabilities, like a broken arm, or aging users as their abilities change.<sup id="cite_ref-WAI_57-0" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-WAI-57"><span>[</span>57<span>]</span></a></sup> The Web receives information as well as providing information and interacting with society. The <a href="http://en.wikipedia.org/wiki/World_Wide_Web_Consortium" title="World Wide Web Consortium">World Wide Web Consortium</a> claims it essential that the Web be accessible, so it can provide equal access and <a href="http://en.wikipedia.org/wiki/Equal_opportunity" title="Equal opportunity">equal opportunity</a> to people with disabilities.<sup id="cite_ref-58" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-58"><span>[</span>58<span>]</span></a></sup> Tim Berners-Lee once noted, "The power of the Web is in its universality. Access by everyone regardless of disability is an essential aspect."<sup id="cite_ref-WAI_57-1" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-WAI-57"><span>[</span>57<span>]</span></a></sup> Many countries regulate <a href="http://en.wikipedia.org/wiki/Web_accessibility" title="Web accessibility">web accessibility</a> as a requirement for websites.<sup id="cite_ref-59" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-59"><span>[</span>59<span>]</span></a></sup> International cooperation in the W3C <a href="http://en.wikipedia.org/wiki/Web_Accessibility_Initiative" title="Web Accessibility Initiative">Web Accessibility Initiative</a> led to simple guidelines that web content authors as well as software developers can use to make the Web accessible to persons who may or may not be using <a href="http://en.wikipedia.org/wiki/Assistive_technology" title="Assistive technology">assistive technology</a>.<sup id="cite_ref-WAI_57-2" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-WAI-57"><span>[</span>57<span>]</span></a></sup><sup id="cite_ref-60" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-60"><span>[</span>60<span>]</span></a></sup> <h2> <span class="mw-headline" id="Internationalization">Internationalization</span></h2> <p> The W3C <a href="http://en.wikipedia.org/wiki/Internationalization_and_localization" title="Internationalization and localization">Internationalization</a> Activity assures that web technology works in all languages, scripts, and cultures.<sup id="cite_ref-61" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-61"><span>[</span>61<span>]</span></a></sup> Beginning in 2004 or 2005, <a href="http://en.wikipedia.org/wiki/Unicode" title="Unicode">Unicode</a> gained ground and eventually in December 2007 surpassed both <a href="http://en.wikipedia.org/wiki/ASCII" title="ASCII">ASCII</a> and Western European as the Web's most frequently used <a href="http://en.wikipedia.org/wiki/Character_encoding" title="Character encoding">character encoding</a>.<sup id="cite_ref-62" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-62"><span>[</span>62<span>]</span></a></sup> Originally <a class="external mw-magiclink-rfc" rel="nofollow" href="http://tools.ietf.org/html/rfc3986">RFC 3986</a> allowed resources to be identified by <a href="http://en.wikipedia.org/wiki/URI" title="URI" class="mw-redirect">URI</a> in a subset of US-ASCII. <a class="external mw-magiclink-rfc" rel="nofollow" href="http://tools.ietf.org/html/rfc3987">RFC 3987</a> allows more characters—any character in the <a href="http://en.wikipedia.org/wiki/Universal_Character_Set" title="Universal Character Set">Universal Character Set</a>—and now a resource can be identified by <a href="http://en.wikipedia.org/wiki/Internationalized_Resource_Identifier" title="Internationalized Resource Identifier" class="mw-redirect">IRI</a> in any language.<sup id="cite_ref-63" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-63"><span>[</span>63<span>]</span></a></sup></p> <h2> <span class="mw-headline" id="Statistics">Statistics</span></h2> <p> Between 2005 and 2010, the number of web users doubled, and was expected to surpass two billion in 2010.<sup id="cite_ref-64" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-64"><span>[</span>64<span>]</span></a></sup> Early studies in 1998 and 1999 estimating the size of the Web using capture/recapture methods showed that much of the web was not indexed by search engines and the Web was much larger than expected.<sup id="cite_ref-65" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-65"><span>[</span>65<span>]</span></a></sup><sup id="cite_ref-66" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-66"><span>[</span>66<span>]</span></a></sup> According to a 2001 study, there were a massive number, over 550 billion, of documents on the Web, mostly in the invisible Web, or <a href="http://en.wikipedia.org/wiki/Deep_Web" title="Deep Web">Deep Web</a>.<sup id="cite_ref-67" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-67"><span>[</span>67<span>]</span></a></sup> A 2002 survey of 2,024 million web pages<sup id="cite_ref-68" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-68"><span>[</span>68<span>]</span></a></sup> determined that by far the most web content was in the English language: 56.4%; next were pages in German (7.7%), French (5.6%), and Japanese (4.9%). A more recent study, which used web searches in 75 different languages to sample the Web, determined that there were over 11.5 billion web pages in the <a href="http://en.wikipedia.org/wiki/Surface_Web" title="Surface Web">publicly indexable web</a> as of the end of January 2005.<sup id="cite_ref-69" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-69"><span>[</span>69<span>]</span></a></sup> As of March 2009, the indexable web contains at least 25.21 billion pages.<sup id="cite_ref-70" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-70"><span>[</span>70<span>]</span></a></sup> On 25 July 2008, Google software engineers Jesse Alpert and Nissan Hajaj announced that <a href="http://en.wikipedia.org/wiki/Google_Search" title="Google Search">Google Search</a> had discovered one trillion unique URLs.<sup id="cite_ref-71" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-71"><span>[</span>71<span>]</span></a></sup> As of May 2009, over 109.5 million domains operated.<sup id="cite_ref-NI_72-0" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-NI-72"><span>[</span>72<span>]</span></a></sup><sup class="noprint Inline-Template" style="white-space:nowrap;">[<i><a href="http://en.wikipedia.org/wiki/Wikipedia:Verifiability" title="Wikipedia:Verifiability"><span title="The material near this tag failed verification of its source citation(s). (November 2011)">not in citation given</span></a></i>]</sup> Of these 74% were commercial or other domains operating in the <code>.com</code> <a href="http://en.wikipedia.org/wiki/Generic_top-level_domain" title="Generic top-level domain">generic top-level domain</a>.<sup id="cite_ref-NI_72-1" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-NI-72"><span>[</span>72<span>]</span></a></sup></p> <p>  </p> <p> Statistics measuring a website's popularity are usually based either on the number of <a href="http://en.wikipedia.org/wiki/Page_view" title="Page view">page views</a> or on associated server '<a href="http://en.wikipedia.org/wiki/Hit_%28internet%29" title="Hit (internet)" class="mw-redirect">hits</a>' (file requests) that it receives.</p> <h2> <span class="mw-headline" id="Speed_issues">Speed issues</span></h2> <p> Frustration over <a href="http://en.wikipedia.org/wiki/Network_congestion" title="Network congestion">congestion</a> issues in the Internet infrastructure and the high <a href="http://en.wikipedia.org/wiki/Latency_%28engineering%29" title="Latency (engineering)">latency</a> that results in slow browsing has led to a pejorative name for the World Wide Web: the <i>World Wide Wait</i>.<sup id="cite_ref-73" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-73"><span>[</span>73<span>]</span></a></sup> </p> <p> Speeding up the Internet is an ongoing discussion over the use of <a href="http://en.wikipedia.org/wiki/Peering" title="Peering">peering</a> and <a href="http://en.wikipedia.org/wiki/Quality_of_service" title="Quality of service">QoS</a> technologies. Other solutions to reduce the congestion can be found at <a href="http://en.wikipedia.org/wiki/W3C" title="W3C" class="mw-redirect">W3C</a>.<sup id="cite_ref-74" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-74"><span>[</span>74<span>]</span></a></sup> <a href="http://en.wikipedia.org/wiki/Guideline" title="Guideline">Guidelines</a> for web response times are:<sup id="cite_ref-75" class="reference"><a href="http://en.wikipedia.org/wiki/World_Wide_Web#cite_note-75"><span>[</span>75<span>]</span></a></sup></p> <ul> <li>0.1 second (one tenth of a second). Ideal response time. The user does not sense any interruption.</li> <li>1 second. Highest acceptable response time. Download times above 1 second interrupt the user experience.</li> <li>10 seconds. Unacceptable response time. The user experience is interrupted and the user is likely to leave the site or system.</li> </ul> <h2> <span class="mw-headline" id="Caching">Caching</span></h2> If a user revisits a web page after a short interval, the browser may not need to re-obtain the page data from the source web server. Almost all web browsers <a href="http://en.wikipedia.org/wiki/Browser_cache" title="Browser cache" class="mw-redirect">cache</a> recently obtained data, usually on the local hard drive. HTTP requests from a browser usually ask only for data that has changed since the last download. If locally cached data is still current, the browser reuses it. Caching reduces the amount of web traffic on the Internet. Decisions about expiration are made independently for each downloaded file, whether image, <a href="http://en.wikipedia.org/wiki/Cascading_Style_Sheets" title="Cascading Style Sheets">stylesheet</a>, <a href="http://en.wikipedia.org/wiki/JavaScript" title="JavaScript">JavaScript</a>, HTML, or other <a href="http://en.wikipedia.org/wiki/Web_resource" title="Web resource">web resource</a>. Thus even on sites with highly dynamic content, many basic resources refresh only occasionally. Web site designers find it worthwhile to collate resources such as CSS data and JavaScript into a few site-wide files so that they can be cached efficiently. This helps reduce page download times and lowers demands on the web server. <p>  </p> <p> There are other components of the Internet that can cache web content. Corporate and academic <a href="http://en.wikipedia.org/wiki/Firewall_%28networking%29" title="Firewall (networking)" class="mw-redirect">firewalls</a> often cache Web resources requested by one user for the benefit of all. (See also <a href="http://en.wikipedia.org/wiki/Web_proxy#Caching" title="Web proxy" class="mw-redirect">caching proxy server</a>.) Some <a href="http://en.wikipedia.org/wiki/Search_engines" title="Search engines" class="mw-redirect">search engines</a> also store cached content from websites. Apart from the facilities built into web servers that can determine when files have been updated and so must be re-sent, designers of dynamically generated web pages can control the HTTP headers sent back to requesting users, so that transient or sensitive pages are not cached. <a href="http://en.wikipedia.org/wiki/Online_banking" title="Online banking">Internet banking</a> and news sites frequently use this facility. Data requested with an <a href="http://en.wikipedia.org/wiki/Hypertext_Transfer_Protocol" title="Hypertext Transfer Protocol">HTTP</a> 'GET' is likely to be cached if other conditions are met; data obtained in response to a 'POST' is assumed to depend on the data that was Posted and so is not cached.</p>

Earth

Earth

From Wikipedia, the free encyclopedia
 

Earth Astronomical symbol of Earth
A planetary disk of white cloud formations, brown and green land masses, and dark blue oceans against a black background. The Arabian peninsula, Africa and Madagascar lie in the upper half of the disk, while Antarctica is at the bottom.
A composite image of Earth produced by NASA.
Orbital characteristics
Epoch J2000.0[n 1]
Aphelion
152098232 km
(1.0167138AU) [n 2]
Perihelion
147098290 km
(0.98329134 AU) [n 2]
149598261 km
(1.00000261 AU) [1]
Eccentricity 0.01671123[1]
365.256363004 d[2]
(1.000017421 yr)
Average orbital speed
29.78 km/s[3]
(107200 km/h)
357.5171deg[3]
Inclination
348.73936 deg[3][n 3]
114.20783 deg[3][n 4]
Satellites
Physical characteristics
Mean radius
6371.0 km[6]
Equatorial radius
6378.1 km[7][8]
Polar radius
6356.8 km[9]
Flattening 0.0033528[10]
Circumference
  • 510072000 km2[13][14][n 5]
  •  (148940000 km2 (29.2%) land
  •   361132000 km2 (70.8%) water)
Volume 1.08321×1012 km3[3]
Mass
5.97219×1024 kg[15]
(3.0×10-6 Suns)
Mean density
5.515 g/cm3[3]
0.3307[17]
11.186 km/s[3]
Sidereal rotation period
0.99726968 d[18]
(23h 56m 4.100s)
Equatorial rotation velocity
1,674.4 km/h (465.1 m/s)[19]
23 deg 26 min 21.4119 s[2]
Albedo
Surface temp. min mean max
Kelvin 184 K[20] 288 K[21] 330 K[22]
Celsius −89.2 °C 15 °C 56.7 °C
Atmosphere
Surface pressure
101.325 kPa (at MSL)
Composition
Earth, also known as the world,[25] Terra,[27] or Gaia,[29] is the third planet from the Sun, the densest planet in the Solar System, the largest of the Solar System's four terrestrial planets, and the only celestial body known to accommodate life. It is home to millions of species,[30] including billions of humans[31] who depend upon its biosphere and minerals. The Earth's human population is divided among about two hundred independent states that interact through diplomacy, conflict, travel, trade, and media.

According to evidence from sources such as radiometric dating, Earth was formed around four and a half billion years ago. Within its first billion years,[32] life appeared in its oceans and began to affect its atmosphere and surface, promoting the proliferation of aerobic as well as anaerobic organisms and causing the formation of the atmosphere's ozone layer. This layer and Earth's magnetic field block the most life-threatening parts of the Sun's radiation, so life was able to flourish on land as well as in water.[33] Since then, Earth's position in the Solar System, its physical properties and its geological history have allowed life to persist.

Earth's lithosphere is divided into several rigid segments, or tectonic plates, that migrate across the surface over periods of many millions of years. Over 70% percent of Earth's surface is covered with water,[34] with the remainder consisting of continents and islands which together have many lakes and other sources of water that contribute to the hydrosphere. Earth's poles are mostly covered with ice that is the solid ice of the Antarctic ice sheet and the sea ice that is the polar ice packs. The planet's interior remains active, with a solid iron inner core, a liquid outer core that generates the magnetic field, and a thick layer of relatively solid mantle.

Earth gravitationally interacts with other objects in space, especially the Sun and the Moon. During one orbit around the Sun, the Earth rotates about its own axis 366.26 times, creating 365.26 solar days, or one sidereal year.[n 6] The Earth's axis of rotation is tilted 23.4° away from the perpendicular of its orbital plane, producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days).[35] The Moon is Earth's only natural satellite. It began orbiting the Earth about 4.53 billion years ago (bya). The Moon's gravitational interaction with Earth stimulates ocean tides, stabilizes the axial tilt, and gradually slows the planet's rotation.

Name and etymology

NASA's 2014 Earth Day "Global Selfie" mosaic, composed of more than 50,000 photographs from around the world.

The modern English Earth developed from a wide variety of Middle English forms,[37] which derived from an Old English noun most often spelled eorðe.[36] It has cognates in every Germanic language and their proto-Germanic root has been reconstructed as *erþō. In its earliest appearances, eorðe was already being used to translate the many senses of Latin terra and Greek γῆ (): the ground,[39] its soil,[41] dry land,[44] the human world,[46] the surface of the world (including the sea),[49] and the globe itself.[51] As with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus among the devotees of Nerthus[52] and later Norse mythology included Jörð, a giantess often given as the mother of Thor.[53]

Originally, earth was written in lowercase and, from early Middle English, its definite sense as "the globe" was expressed as the earth. By early Modern English, many nouns were capitalized and the earth became (and often remained) the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is simply given as Earth, by analogy with the names of the other planets.[36] House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes Earth when appearing as a name (e.g., "Earth's atmosphere") but writes it in lowercase when preceded by the (e.g., "the atmosphere of the earth"). It almost always appears in lowercase in colloquial expressions such as "what on earth are you doing?"[54]

Composition and structure

Earth is a terrestrial planet, meaning that it is a rocky body, rather than a gas giant like Jupiter. It is the largest of the four terrestrial planets in size and mass. Of these four planets, Earth also has the highest density, the highest surface gravity, the strongest magnetic field, and fastest rotation,[55] and is probably the only one with active plate tectonics.[56]

Shape

Stratocumulus clouds over the Pacific, viewed from orbit

The shape of the Earth approximates an oblate spheroid, a sphere flattened along the axis from pole to pole such that there is a bulge around the equator.[57] This bulge results from the rotation of the Earth, and causes the diameter at the equator to be 43 km (kilometer) larger than the pole-to-pole diameter.[58] For this reason the furthest point on the surface from the Earth's center of mass is the Chimborazo volcano in Ecuador.[59] The average diameter of the reference spheroid is about 12742 km, which is approximately 40,000 km/π, as the meter was originally defined as 1/10,000,000 of the distance from the equator to the North Pole through Paris, France.[60]

Local topography deviates from this idealized spheroid, although on a global scale, these deviations are small: Earth has a tolerance of about one part in about 584, or 0.17%, from the reference spheroid, which is less than the 0.22% tolerance allowed in billiard balls.[61] The largest local deviations in the rocky surface of the Earth are Mount Everest (8,848 m above local sea level) and the Mariana Trench (10911 m below local sea level). Due to the equatorial bulge, the surface locations farthest from the center of the Earth are the summits of Mount Chimborazo in Ecuador and Huascarán in Peru.[62][63][64]
Chemical composition of the crust[65]
Compound Formula Composition
Continental Oceanic
silica SiO2 60.2% 48.6%
alumina Al2O3 15.2% 16.5%
lime CaO 5.5% 12.3%
magnesia MgO 3.1% 6.8%
iron(II) oxide FeO 3.8% 6.2%
sodium oxide Na2O 3.0% 2.6%
potassium oxide K2O 2.8% 0.4%
iron(III) oxide Fe2O3 2.5% 2.3%
water H2O 1.4% 1.1%
carbon dioxide CO2 1.2% 1.4%
titanium dioxide TiO2 0.7% 1.4%
phosphorus pentoxide P2O5 0.2% 0.3%
Total 99.6% 99.9%

Chemical composition

The mass of the Earth is approximately 5.98×1024 kg. It is composed mostly of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%); with the remaining 1.2% consisting of trace amounts of other elements. Due to mass segregation, the core region is believed to be primarily composed of iron (88.8%), with smaller amounts of nickel (5.8%), sulfur (4.5%), and less than 1% trace elements.[66]
The geochemist F. W. Clarke calculated that a little more than 47% of the Earth's crust consists of oxygen. The more common rock constituents of the Earth's crust are nearly all oxides; chlorine, sulfur and fluorine are the only important exceptions to this and their total amount in any rock is usually much less than 1%. The principal oxides are silica, alumina, iron oxides, lime, magnesia, potash and soda. The silica functions principally as an acid, forming silicates, and all the commonest minerals of igneous rocks are of this nature. From a computation based on 1,672 analyses of all kinds of rocks, Clarke deduced that 99.22% were composed of 11 oxides (see the table at right), with the other constituents occurring in minute quantities.[67]

Internal structure

The interior of the Earth, like that of the other terrestrial planets, is divided into layers by their chemical or physical (rheological) properties, but unlike the other terrestrial planets, it has a distinct outer and inner core. The outer layer of the Earth is a chemically distinct silicate solid crust, which is underlain by a highly viscous solid mantle. The crust is separated from the mantle by the Mohorovičić discontinuity, and the thickness of the crust varies: averaging km (kilometers) under the oceans and 30-50 km on the continents. The crust and the cold, rigid, top of the upper mantle are collectively known as the lithosphere, and it is of the lithosphere that the tectonic plates are comprised. Beneath the lithosphere is the asthenosphere, a relatively low-viscosity layer on which the lithosphere rides. Important changes in crystal structure within the mantle occur at 410 and 660 km below the surface, spanning a transition zone that separates the upper and lower mantle. Beneath the mantle, an extremely low viscosity liquid outer core lies above a solid inner core.[68] The inner core may rotate at a slightly higher angular velocity than the remainder of the planet, advancing by 0.1–0.5° per year.[69]
Geologic layers of the Earth[70]
Earth-crust-cutaway-english.svg

Earth cutaway from core to exosphere. Not to scale.
Depth[71]
km
Component Layer Density
g/cm3
0–60 Lithosphere[n 7]
0–35 Crust[n 8] 2.2–2.9
35–60 Upper mantle 3.4–4.4
  35–2890 Mantle 3.4–5.6
100–700 Asthenosphere
2890–5100 Outer core 9.9–12.2
5100–6378 Inner core 12.8–13.1

Heat

Earth's internal heat comes from a combination of residual heat from planetary accretion (about 20%) and heat produced through radioactive decay (80%).[72] The major heat-producing isotopes in Earth are potassium-40, uranium-238, uranium-235, and thorium-232.[73] At the center, the temperature may be up to 6,000 °C (10,830 °F),[74] and the pressure could reach 360 GPa.[75] Because much of the heat is provided by radioactive decay, scientists believe that early in Earth's history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. This extra heat production, twice present-day at approximately byr,[72] would have increased temperature gradients within Earth, increasing the rates of mantle convection and plate tectonics, and allowing the production of igneous rocks such as komatiites that are not formed today.[76]

Present-day major heat-producing isotopes[77]
Isotope Heat release
W/kg isotope
Half-life

years
Mean mantle concentration
kg isotope/kg mantle
Heat release
W/kg mantle
238U 9.46 × 10−5 4.47 × 109 30.8 × 10−9 2.91 × 10−12
235U 5.69 × 10−4 7.04 × 108 0.22 × 10−9 1.25 × 10−13
232Th 2.64 × 10−5 1.40 × 1010 124 × 10−9 3.27 × 10−12
40K 2.92 × 10−5 1.25 × 109 36.9 × 10−9 1.08 × 10−12

The mean heat loss from Earth is 87 mW m−2, for a global heat loss of 4.42 × 1013 W.[78] A portion of the core's thermal energy is transported toward the crust by mantle plumes; a form of convection consisting of upwellings of higher-temperature rock. These plumes can produce hotspots and flood basalts.[79] More of the heat in Earth is lost through plate tectonics, by mantle upwelling associated with mid-ocean ridges. The final major mode of heat loss is through conduction through the lithosphere, the majority of which occurs in the oceans because the crust there is much thinner than that of the continents.[80]

Tectonic plates

Earth's main plates[81]
Shows the extent and boundaries of tectonic plates, with superimposed outlines of the continents they support
Plate name Area
106 km2
103.3
78.0
75.9
67.8
60.9
47.2
43.6
The mechanically rigid outer layer of the Earth, the lithosphere, is broken into pieces called tectonic plates. These plates are rigid segments that move in relation to one another at one of three types of plate boundaries: Convergent boundaries, at which two plates come together, Divergent boundaries, at which two plates are pulled apart, and Transform boundaries, in which two plates slide past one another laterally. Earthquakes, volcanic activity, mountain-building, and oceanic trench formation can occur along these plate boundaries.[82] The tectonic plates ride on top of the asthenosphere, the solid but less-viscous part of the upper mantle that can flow and move along with the plates,[83] and their motion is strongly coupled with convection patterns inside the Earth's mantle.
As the tectonic plates migrate across the planet, the ocean floor is subducted under the leading edges of the plates at convergent boundaries. At the same time, the upwelling of mantle material at divergent boundaries creates mid-ocean ridges. The combination of these processes continually recycles the oceanic crust back into the mantle. Due to this recycling, most of the ocean floor is less than 100 myr old in age. The oldest oceanic crust is located in the Western Pacific, and has an estimated age of about 200 myr.[84][85] By comparison, the oldest dated continental crust is 4030 myr.[86]

The seven major plates are the Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American. Other notable plates include the Arabian Plate, the Caribbean Plate, the Nazca Plate off the west coast of South America and the Scotia Plate in the southern Atlantic Ocean. The Australian Plate fused with the Indian Plate between 50 and 55 mya. The fastest-moving plates are the oceanic plates, with the Cocos Plate advancing at a rate of 75 mm/year[87] and the Pacific Plate moving 52–69 mm/year. At the other extreme, the slowest-moving plate is the Eurasian Plate, progressing at a typical rate of about 21 mm/year.[88]

Surface


Circle frame.svg
Features of Earth's solid surface shown as percentages of the planet's total surface area
  Oceanic ridges (22.1%)
  Ocean basin floors (29.8%)
  Continental mountains (10.3%)
  Continental lowlands (18.9%)
  Continental shelves and slopes (11.4%)
  Continental rise (3.8%)
  Volcanic island arcs, trenches, submarine volcanoes, and hills (3.7%)

The Earth's terrain varies greatly from place to place. About 70.8%[13] of the surface is covered by water, with much of the continental shelf below sea level. This equates to 361.132 million km2 (139.43 million sq mi).[89] The submerged surface has mountainous features, including a globe-spanning mid-ocean ridge system, as well as undersea volcanoes,[58] oceanic trenches, submarine canyons, oceanic plateaus and abyssal plains. The remaining 29.2% (148.94 million km2, or 57.51 million sq mi) not covered by water consists of mountains, deserts, plains, plateaus, and other geomorphologies.

The planetary surface undergoes reshaping over geological time periods due to tectonics and erosion. The surface features built up or deformed through plate tectonics are subject to steady weathering from precipitation, thermal cycles, and chemical effects. Glaciation, coastal erosion, the build-up of coral reefs, and large meteorite impacts[90] also act to reshape the landscape.

The continental crust consists of lower density material such as the igneous rocks granite and andesite. Less common is basalt, a denser volcanic rock that is the primary constituent of the ocean floors.[91] Sedimentary rock is formed from the accumulation of sediment that becomes compacted together. Nearly 75% of the continental surfaces are covered by sedimentary rocks, although they form only about 5% of the crust.[92] The third form of rock material found on Earth is metamorphic rock, which is created from the transformation of pre-existing rock types through high pressures, high temperatures, or both. The most abundant silicate minerals on the Earth's surface include quartz, the feldspars, amphibole, mica, pyroxene and olivine.[93] Common carbonate minerals include calcite (found in limestone) and dolomite.[94]

The pedosphere is the outermost layer of the Earth that is composed of soil and subject to soil formation processes. It exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere. Currently the total arable land is 13.31% of the land surface, with only 4.71% supporting permanent crops.[14] Close to 40% of the Earth's land surface is presently used for cropland and pasture, or an estimated 1.3×107 km2 of cropland and 3.4×107 km2 of pastureland.[95]

The elevation of the land surface of the Earth varies from the low point of −418 m at the Dead Sea, to a 2005-estimated maximum altitude of 8,848 m at the top of Mount Everest. The mean height of land above sea level is 840 m.[96]

Besides being divided logically into Northern and Southern Hemispheres centered on the earths poles, the earth has been divided arbitrarily into Eastern and Western Hemispheres. The surface of the Earth is traditionally divided into seven continents and various seas. As people settled and organized the planet, nearly all the land was divided into nations. As of 2013, there are about 196 recognized nations.[97] An example of how major geographical regions can be broken down is Africa, America, Antarctica, Asia, Australia, and Europe.

Hydrosphere

Elevation histogram of the surface of the Earth

The abundance of water on Earth's surface is a unique feature that distinguishes the "Blue Planet" from others in the Solar System. The Earth's hydrosphere consists chiefly of the oceans, but technically includes all water surfaces in the world, including inland seas, lakes, rivers, and underground waters down to a depth of 2,000 m. The deepest underwater location is Challenger Deep of the Mariana Trench in the Pacific Ocean with a depth of 10,911.4 m.[n 10][98]

The mass of the oceans is approximately 1.35×1018 metric tons, or about 1/4400 of the total mass of the Earth. The oceans cover an area of 3.618×108 km2 with a mean depth of 3682 m, resulting in an estimated volume of 1.332×109 km3.[99] If all the land on Earth were spread evenly, water would rise to an altitude of more than 2.7 km.[n 11] About 97.5% of the water is saline, while the remaining 2.5% is fresh water. Most fresh water, about 68.7%, is currently ice.[100]

The average salinity of the Earth's oceans is about 35 grams of salt per kilogram of sea water (3.5% salt).[101] Most of this salt was released from volcanic activity or extracted from cool, igneous rocks.[102] The oceans are also a reservoir of dissolved atmospheric gases, which are essential for the survival of many aquatic life forms.[103] Sea water has an important influence on the world's climate, with the oceans acting as a large heat reservoir.[104] Shifts in the oceanic temperature distribution can cause significant weather shifts, such as the El Niño-Southern Oscillation.[105]

Atmosphere

The atmospheric pressure on the surface of the Earth averages 101.325 kPa, with a scale height of about 8.5 km.[3] It is 78% nitrogen and 21% oxygen, with trace amounts of water vapor, carbon dioxide and other gaseous molecules. The height of the troposphere varies with latitude, ranging between 8 km at the poles to 17 km at the equator, with some variation resulting from weather and seasonal factors.[106]
Earth's biosphere has significantly altered its atmosphere. Oxygenic photosynthesis evolved 2.7 bya, forming the primarily nitrogen–oxygen atmosphere of today.[107] This change enabled the proliferation of aerobic organisms as well as the formation of the ozone layer which blocks ultraviolet solar radiation, permitting life on land. Other atmospheric functions important to life on Earth include transporting water vapor, providing useful gases, causing small meteors to burn up before they strike the surface, and moderating temperature.[108] This last phenomenon is known as the greenhouse effect: trace molecules within the atmosphere serve to capture thermal energy emitted from the ground, thereby raising the average temperature. Water vapor, carbon dioxide, methane and ozone are the primary greenhouse gases in the Earth's atmosphere. Without this heat-retention effect, the average surface would be −18 °C, in contrast to the current +15 °C, and life would likely not exist.[109]

Weather and climate


The Earth's atmosphere has no definite boundary, slowly becoming thinner and fading into outer space. Three-quarters of the atmosphere's mass is contained within the first 11 km of the planet's surface. This lowest layer is called the troposphere. Energy from the Sun heats this layer, and the surface below, causing expansion of the air. This lower-density air then rises, and is replaced by cooler, higher-density air. The result is atmospheric circulation that drives the weather and climate through redistribution of thermal energy.[110]

The primary atmospheric circulation bands consist of the trade winds in the equatorial region below 30° latitude and the westerlies in the mid-latitudes between 30° and 60°.[111] Ocean currents are also important factors in determining climate, particularly the thermohaline circulation that distributes thermal energy from the equatorial oceans to the polar regions.[112]

Water vapor generated through surface evaporation is transported by circulatory patterns in the atmosphere. When atmospheric conditions permit an uplift of warm, humid air, this water condenses and settles to the surface as precipitation.[110] Most of the water is then transported to lower elevations by river systems and usually returned to the oceans or deposited into lakes. This water cycle is a vital mechanism for supporting life on land, and is a primary factor in the erosion of surface features over geological periods. Precipitation patterns vary widely, ranging from several meters of water per year to less than a millimeter. Atmospheric circulation, topological features and temperature differences determine the average precipitation that falls in each region.[113]

The amount of solar energy reaching the Earth's decreases with increasing latitude. At higher latitudes the sunlight reaches the surface at lower angles and it must pass through thicker columns of the atmosphere. As a result, the mean annual air temperature at sea level decreases by about 0.4 °C per degree of latitude away from the equator.[114] The Earth can be subdivided into specific latitudinal belts of approximately homogeneous climate. Ranging from the equator to the polar regions, these are the tropical (or equatorial), subtropical, temperate and polar climates.[115] Climate can also be classified based on the temperature and precipitation, with the climate regions characterized by fairly uniform air masses. The commonly used Köppen climate classification system (as modified by Wladimir Köppen's student Rudolph Geiger) has five broad groups (humid tropics, arid, humid middle latitudes, continental and cold polar), which are further divided into more specific subtypes.[111]

Upper atmosphere

This view from orbit shows the full Moon partially obscured and deformed by the Earth's atmosphere. NASA image

Above the troposphere, the atmosphere is usually divided into the stratosphere, mesosphere, and thermosphere.[108] Each layer has a different lapse rate, defining the rate of change in temperature with height. Beyond these, the exosphere thins out into the magnetosphere, where the Earth's magnetic fields interact with the solar wind.[116] Within the stratosphere is the ozone layer, a component that partially shields the surface from ultraviolet light and thus is important for life on Earth. The Kármán line, defined as 100 km above the Earth's surface, is a working definition for the boundary between atmosphere and space.[117]

Thermal energy causes some of the molecules at the outer edge of the Earth's atmosphere to increase their velocity to the point where they can escape from the planet's gravity. This causes a slow but steady leakage of the atmosphere into space. Because unfixed hydrogen has a low molecular weight, it can achieve escape velocity more readily and it leaks into outer space at a greater rate than other gasses.[118] The leakage of hydrogen into space contributes to the pushing of the Earth from an initially reducing state to its current oxidizing one. Photosynthesis provided a source of free oxygen, but the loss of reducing agents such as hydrogen is believed to have been a necessary precondition for the widespread accumulation of oxygen in the atmosphere.[119] Hence the ability of hydrogen to escape from the Earth's atmosphere may have influenced the nature of life that developed on the planet.[120] In the current, oxygen-rich atmosphere most hydrogen is converted into water before it has an opportunity to escape. Instead, most of the hydrogen loss comes from the destruction of methane in the upper atmosphere.[121]

Magnetic field

Diagram showing the magnetic field lines of the Earth's magnetosphere. The lines are swept back in the anti-solar direction under the influence of the solar wind.
Schematic of Earth's magnetosphere. The solar wind flows from left to right

The main part of theEarth's magnetic field is generated in the Earth's core, the site of a dynamo process that converts kinetic energy of fluid convective motion into electromagnetic energy. The field extends outwards from the core, through the mantle, and up to the Earth's surface, where it is, to rough approximation, a dipole. The poles of the dipole are presently located close to the Earth's geographic poles. At the equator of the magnetic field, the magnetic field strength at the planet's surface is 3.05 × 10−5 T, with global magnetic dipole moment of 7.91 × 1015 T m3.[122] The convection movements in the core are chaotic; the magnetic poles drift and periodically change alignment. This causes field reversals at irregular intervals averaging a few times every million years.
The most recent reversal occurred approximately 700,000 years ago.[123][124]

Magnetosphere

The field forms the magnetosphere, which deflects particles in the solar wind. The sunward edge of the bow shock is located at about 13 times the radius of the Earth. The collision between the magnetic field and the solar wind forms the Van Allen radiation belts, a pair of concentric, torus-shaped regions of energetic charged particles. When the plasma enters the Earth's atmosphere at the magnetic poles, it forms the aurora.[125]

Orbit and rotation

Rotation

Earth's axial tilt (or obliquity) and its relation to the rotation axis and plane of orbit

Earth's rotation period relative to the Sun—its mean solar day—is 86,400 seconds of mean solar time (86,400.0025 SI seconds).[126] As the Earth's solar day is now slightly longer than it was during the 19th century due to tidal acceleration, each day varies between 0 and 2 SI ms longer.[127][128]
Earth's rotation period relative to the fixed stars, called its stellar day by the International Earth Rotation and Reference Systems Service (IERS), is 86,164.098903691 seconds of mean solar time (UT1), or 23h 56m 4.098903691s.[2][n 12] Earth's rotation period relative to the precessing or moving mean vernal equinox, misnamed its sidereal day, is 86,164.09053083288 seconds of mean solar time (UT1) (23h 56m 4.09053083288s) as of 1982.[2] Thus the sidereal day is shorter than the stellar day by about 8.4 ms.[129] The length of the mean solar day in SI seconds is available from the IERS for the periods 1623–2005[130] and 1962–2005.[131]

Apart from meteors within the atmosphere and low-orbiting satellites, the main apparent motion of celestial bodies in the Earth's sky is to the west at a rate of 15°/h = 15'/min. For bodies near the celestial equator, this is equivalent to an apparent diameter of the Sun or Moon every two minutes; from the planet's surface, the apparent sizes of the Sun and the Moon are approximately the same.[132][133]

Orbit

Earth orbits the Sun at an average distance of about 150 million kilometers every 365.2564 mean solar days, or one sidereal year. From Earth, this gives an apparent movement of the Sun eastward with respect to the stars at a rate of about 1°/day, which is one apparent Sun or Moon diameter every 12 hours. Due to this motion, on average it takes 24 hours—a solar day—for Earth to complete a full rotation about its axis so that the Sun returns to the meridian. The orbital speed of the Earth averages about 29.8 km/s (107,000 km/h), which is fast enough to travel a distance equal to the planet's diameter, about 12,742 km, in seven minutes, and the distance to the Moon, 384,000 km, in about 3.5 hours.[3]
The Moon revolves with the Earth around a common barycenter every 27.32 days relative to the background stars. When combined with the Earth–Moon system's common revolution around the Sun, the period of the synodic month, from new moon to new moon, is 29.53 days. Viewed from the celestial north pole, the motion of Earth, the Moon and their axial rotations are all counterclockwise. Viewed from a vantage point above the north poles of both the Sun and the Earth, the Earth revolves in a counterclockwise direction about the Sun. The orbital and axial planes are not precisely aligned: Earth's axis is tilted some 23.4 degrees from the perpendicular to the Earth–Sun plane (the ecliptic), and the Earth–Moon plane is tilted up to ±5.1 degrees against the Earth–Sun plane. Without this tilt, there would be an eclipse every two weeks, alternating between lunar eclipses and solar eclipses.[3][134]

The Hill sphere, or gravitational sphere of influence, of the Earth is about 1.5 Gm or 1,500,000 km in radius.[135][n 13] This is the maximum distance at which the Earth's gravitational influence is stronger than the more distant Sun and planets. Objects must orbit the Earth within this radius, or they can become unbound by the gravitational perturbation of the Sun.

Earth, along with the Solar System, is situated in the Milky Way galaxy and orbits about 28,000 light years from the center of the galaxy. It is currently about 20 light years above the galactic plane in the Orion spiral arm.[136]

Axial tilt and seasons

Due to the axial tilt of the Earth, the amount of sunlight reaching any given point on the surface varies over the course of the year. This causes seasonal change in climate, with summer in the northern hemisphere occurring when the North Pole is pointing toward the Sun, and winter taking place when the pole is pointed away. During the summer, the day lasts longer and the Sun climbs higher in the sky. In winter, the climate becomes generally cooler and the days shorter. Above the Arctic Circle, an extreme case is reached where there is no daylight at all for part of the year—a polar night. In the southern hemisphere the situation is exactly reversed, with the South Pole oriented opposite the direction of the North Pole.
By astronomical convention, the four seasons are determined by the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere, Winter Solstice occurs on about December 21, Summer Solstice is near June 21, Spring Equinox is around March 20 and Autumnal Equinox is about September 23. In the Southern hemisphere, the situation is reversed, with the Summer and Winter Solstices exchanged and the Spring and Autumnal Equinox dates switched.[137]
NASA's Cassini spacecraft photographs the Earth and Moon (visible bottom-right) from Saturn (July 19, 2013).

The angle of the Earth's tilt is relatively stable over long periods of time. The tilt does undergo nutation; a slight, irregular motion with a main period of 18.6 years.[138] The orientation (rather than the angle) of the Earth's axis also changes over time, precessing around in a complete circle over each 25,800 year cycle; this precession is the reason for the difference between a sidereal year and a tropical year. Both of these motions are caused by the varying attraction of the Sun and Moon on the Earth's equatorial bulge. From the perspective of the Earth, the poles also migrate a few meters across the surface. This polar motion has multiple, cyclical components, which collectively are termed quasiperiodic motion. In addition to an annual component to this motion, there is a 14-month cycle called the Chandler wobble. The rotational velocity of the Earth also varies in a phenomenon known as length of day variation.[139]

In modern times, Earth's perihelion occurs around January 3, and the aphelion around July 4. These dates change over time due to precession and other orbital factors, which follow cyclical patterns known as Milankovitch cycles. The changing Earth–Sun distance causes an increase of about 6.9%[n 14] in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.[140]

Habitability

This ancient impact crater, now filled with water, marks Earth's surface

A planet that can sustain life is termed habitable, even if life did not originate there. The Earth provides liquid water—an environment where complex organic molecules can assemble and interact, and sufficient energy to sustain metabolism.[141] The distance of the Earth from the Sun, as well as its orbital eccentricity, rate of rotation, axial tilt, geological history, sustaining atmosphere and protective magnetic field all contribute to the current climatic conditions at the surface.[142]

Biosphere

Coral reef and beach

A planet's life forms are sometimes said to form a "biosphere". The Earth's biosphere is generally believed to have begun evolving about 3.5 bya.[107] The biosphere is divided into a number of biomes, inhabited by broadly similar plants and animals. On land, biomes are separated primarily by differences in latitude, height above sea level and humidity. Terrestrial biomes lying within the Arctic or Antarctic Circles, at high altitudes or in extremely arid areas are relatively barren of plant and animal life; species diversity reaches a peak in humid lowlands at equatorial latitudes.[143]

Evolution of life

Speculative phylogenetic tree of life on Earth based on rRNA analysis.

Highly energetic chemistry is thought to have produced a self-replicating molecule around bya and half a billion years later the last common ancestor of all life existed.[144] The development of photosynthesis allowed the Sun's energy to be harvested directly by life forms; the resultant oxygen accumulated in the atmosphere and formed a layer of ozone (a form of molecular oxygen [O3]) in the upper atmosphere.[107] The incorporation of smaller cells within larger ones resulted in the development of complex cells called eukaryotes.[145] True multicellular organisms formed as cells within colonies became increasingly specialized. Aided by the absorption of harmful ultraviolet radiation by the ozone layer, life colonized the surface of Earth.[146] The earliest evidences for life on Earth are graphite found to be biogenic in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland[147] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.[148][149]

Since the 1960s, it has been hypothesized that severe glacial action between 750 and 580 mya, during the Neoproterozoic, covered much of the planet in a sheet of ice. This hypothesis has been termed "Snowball Earth", and is of particular interest because it preceded the Cambrian explosion, when multicellular life forms began to proliferate.[150]

Following the Cambrian explosion, about 535 mya, there have been five major mass extinctions.[151] The most recent such event was 66 mya, when an asteroid impact triggered the extinction of the (non-avian) dinosaurs and other large reptiles, but spared some small animals such as mammals, which then resembled shrews. Over the past 66 myr, mammalian life has diversified, and several million years ago an African ape-like animal such as Orrorin tugenensis gained the ability to stand upright.[152] This enabled tool use and encouraged communication that provided the nutrition and stimulation needed for a larger brain, which allowed the evolution of the human race. The development of agriculture, and then civilization, allowed humans to influence the Earth in a short time span as no other life form had,[153] affecting both the nature and quantity of other life forms.

Natural resources and land use

The Earth provides resources that are exploitable by humans for useful purposes. Some of these are non-renewable resources, such as mineral fuels, that are difficult to replenish on a short time scale. Large deposits of fossil fuels are obtained from the Earth's crust, consisting of coal, petroleum, natural gas and methane clathrate. These deposits are used by humans both for energy production and as feedstock for chemical production. Mineral ore bodies have also been formed in Earth's crust through a process of ore genesis, resulting from actions of erosion and plate tectonics.[155] These bodies form concentrated sources for many metals and other useful elements.

The Earth's biosphere produces many useful biological products for humans, including (but far from limited to) food, wood, pharmaceuticals, oxygen, and the recycling of many organic wastes. The land-based ecosystem depends upon topsoil and fresh water, and the oceanic ecosystem depends upon dissolved nutrients washed down from the land.[156] In 1980, 5,053 Mha (50.53 million km2) of the Earth's land surface consisted of forest and woodlands, 6,788 Mha (67.88 million km2) was grasslands and pasture, and 1,501 Mha (15.01 million km2) was cultivated as croplands.[157] The estimated amount of irrigated land in 1993 was 2,481,250 square kilometres (958,020 sq mi).[14] Humans also live on the land by using building materials to construct shelters.

Natural and environmental hazards

Large areas of the Earth's surface are subject to extreme weather such as tropical cyclones, hurricanes, or typhoons that dominate life in those areas. From 1980 to 2000, these events caused an average of 11,800 deaths per year.[158] Many places are subject to earthquakes, landslides, tsunamis, volcanic eruptions, tornadoes, sinkholes, blizzards, floods, droughts, wildfires, and other calamities and disasters.

Many localized areas are subject to human-made pollution of the air and water, acid rain and toxic substances, loss of vegetation (overgrazing, deforestation, desertification), loss of wildlife, species extinction, soil degradation, soil depletion, erosion, and introduction of invasive species.

According to the United Nations, a scientific consensus exists linking human activities to global warming due to industrial carbon dioxide emissions. This is predicted to produce changes such as the melting of glaciers and ice sheets, more extreme temperature ranges, significant changes in weather and a global rise in average sea levels.[159]

Human geography

The seven continents of Earth[160]
A composite picture consisting of DMSP/OLS ground-illumination data for 2000 placed on a simulated night-time image of Earth.

Cartography, the study and practice of map-making, and geography, the study of the lands, features, inhabitants and phenomena on Earth, have historically been the disciplines devoted to depicting the Earth. Surveying, the determination of locations and distances, and to a lesser extent navigation, the determination of position and direction, have developed alongside cartography and geography, providing and suitably quantifying the requisite information.

Earth has reached approximately seven billion human inhabitants as of October 31, 2011.[161] Projections indicate that the world's human population will reach 9.2 billion in 2050.[162] Most of the growth is expected to take place in developing nations. Human population density varies widely around the world, but a majority live in Asia. By 2020, 60% of the world's population is expected to be living in urban, rather than rural, areas.[163]

It is estimated that only one-eighth of the surface of the Earth is suitable for humans to live on: three-quarters is covered by oceans, while half of the land area is either desert (14%),[164] high mountains (27%),[165] or other unsuitable terrain. The northernmost permanent settlement in the world is Alert, on Ellesmere Island in Nunavut, Canada.[166] (82°28′N) The southernmost is the Amundsen-Scott South Pole Station, in Antarctica, almost exactly at the South Pole. (90°S)

Independent sovereign nations claim the planet's entire land surface, except for some parts of Antarctica and the odd unclaimed area of Bir Tawil between Egypt and Sudan. As of 2013, there are 206 sovereign states, including the 193 United Nations member states. In addition, there are 59 dependent territories, and a number of autonomous areas, territories under dispute and other entities.[14] Historically, Earth has never had a sovereign government with authority over the entire globe, although a number of nation-states have striven for world domination and failed.[167]

The United Nations is a worldwide intergovernmental organization that was created with the goal of intervening in the disputes between nations, thereby avoiding armed conflict.[168] The U.N. serves primarily as a forum for international diplomacy and international law. When the consensus of the membership permits, it provides a mechanism for armed intervention.[169]
The first "earthrise" ever seen directly by humans, photographed by astronauts on board Apollo 8.

The first human to orbit the Earth was Yuri Gagarin on April 12, 1961.[170] In total, about 487 people have visited outer space and reached Earth orbit as of July 30, 2010, and, of these, twelve have walked on the Moon.[171][172][173] Normally the only humans in space are those on the International Space Station. The station's crew, currently six people, is usually replaced every six months.[174] The furthest humans have travelled from Earth is 400,171 km, achieved during the Apollo 13 mission in 1970.[175]

Cultural and historical viewpoint

The standard astronomical symbol of the Earth consists of a cross circumscribed by a circle, Earth symbol.svg.[176] Unlike the rest of the planets in the Solar System, humankind did not begin to view the Earth as a moving object in orbit around the Sun until the 16th century.[177] Earth has often been personified as a deity, in particular a goddess. In many cultures a mother goddess is also portrayed as a fertility deity. Creation myths in many religions recall a story involving the creation of the Earth by a supernatural deity or deities. A variety of religious groups, often associated with fundamentalist branches of Protestantism[178] or Islam,[179] assert that their interpretations of these creation myths in sacred texts are literal truth and should be considered alongside or replace conventional scientific accounts of the formation of the Earth and the origin and development of life.[180] Such assertions are opposed by the scientific community[181][182] and by other religious groups.[183][184][185] A prominent example is the creation–evolution controversy.

In the past, there were varying levels of belief in a flat Earth,[186] but this was displaced by spherical Earth, a concept that has been credited to Pythagoras (6th century BC).[187] Human cultures have developed many views of the planet, including its personification as a planetary deity, its shape as flat, its position as the center of the universe, and in the modern Gaia Principle, as a single, self-regulating organism in its own right.

Chronology

Formation

Artist's impression of the birth of the Solar System

The earliest material found in the Solar System is dated to 4.5672±0.0006 billion years ago (bya);[188] therefore, it is inferred that the Earth must have been formed by accretion around this time. By 4.54±0.04 bya[32] the primordial Earth had formed. The formation and evolution of the Solar System bodies occurred in tandem with the Sun. In theory a solar nebula partitions a volume out of a molecular cloud by gravitational collapse, which begins to spin and flatten into a circumstellar disk, and then the planets grow out of that in tandem with the star. A nebula contains gas, ice grains and dust (including primordial nuclides). In nebular theory planetesimals commence forming as particulate accrues by cohesive clumping and then by gravity. The assembly of the primordial Earth proceeded for 10–20 myr.[189] The Moon formed shortly thereafter, about 4.53 bya.[190]

The formation of the Moon remains a topic of debate. The working hypothesis is that it formed by accretion from material loosed from the Earth after a Mars-sized object, named Theia, impacted with Earth.[191] The model, however, is not self-consistent. In this scenario, the mass of Theia is 10% of the Earth's mass,[192] it impacts with the Earth in a glancing blow,[193] and some of its mass merges with the Earth. Between approximately 3.8 and 4.1 bya, numerous asteroid impacts during the Late Heavy Bombardment caused significant changes to the greater surface environment of the Moon, and by inference, to the Earth.

Geological history

Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapor. The origin of the world's oceans was condensation augmented by water and ice delivered by asteroids, proto-planets, and comets.[194] In this model, atmospheric "greenhouse gases" kept the oceans from freezing while the newly forming Sun was only at 70% luminosity.[195] By 3.5 bya, the Earth's magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.[196] A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapor began to act in the atmosphere. The two models[197] that explain land mass propose either a steady growth to the present-day forms[198] or, more likely, a rapid growth[199] early in Earth history[200] followed by a long-term steady continental area.[201][202][203]
Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times. Roughly 750 mya (million years ago), one of the earliest known supercontinents, Rodinia, began to break apart. The continents later recombined to form Pannotia, 600–540 mya, then finally Pangaea, which also broke apart 180 mya.[204]

The present pattern of ice ages began about 40 mya and then intensified during the Pleistocene about 3 mya. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100000 years. The last continental glaciation ended 10,000 years ago.[205]

Predicted future

Estimates on how much longer the planet will be able to continue to support life range from 500 million years (myr), to as long as 2.3 billion years (byr).[206][207][208] The future of the planet is closely tied to that of the Sun. As a result of the steady accumulation of helium at the Sun's core, the star's total luminosity will slowly increase. The luminosity of the Sun will grow by 10% over the next 1.1 byr and by 40% over the next 3.5 byr.[209] Climate models indicate that the rise in radiation reaching the Earth is likely to have dire consequences, including the loss of the planet's oceans.[210]
The Earth's increasing surface temperature will accelerate the inorganic CO2 cycle, reducing its concentration to levels lethally low for plants (10 ppm for C4 photosynthesis) in approximately 500-900 myr.[206] The lack of vegetation will result in the loss of oxygen in the atmosphere, so animal life will become extinct within several million more years.[211] After another billion years all surface water will have disappeared[207] and the mean global temperature will reach 70 °C[211] (158 °F). The Earth is expected to be effectively habitable for about another 500 myr from that point,[206] although this may be extended up to 2.3 byr if the nitrogen is removed from the atmosphere.[208] Even if the Sun were eternal and stable, 27% of the water in the modern oceans will descend to the mantle in one billion years, due to reduced steam venting from mid-ocean ridges.[212]
14 billion year timeline showing Sun's present age at 4.6 byr; from 6 byr Sun gradually warming, becoming a red dwarf at 10 byr, "soon" followed by its transformation into a white dwarf star
Life cycle of the Sun

The Sun, as part of its evolution, will become a red giant in about 5 byr. Models predict that the Sun will expand to roughly 1 AU (150,000,000 km), which is about 250 times its present radius.[209][213] Earth's fate is less clear. As a red giant, the Sun will lose roughly 30% of its mass, so, without tidal effects, the Earth will move to an orbit 1.7 AU (250,000,000 km) from the Sun, when the star reaches its maximum radius. The planet was, therefore, initially expected to escape envelopment by the expanded Sun's sparse outer atmosphere, though most, if not all, remaining life would have been destroyed by the Sun's increased luminosity (peaking at about 5,000 times its present level).[209] A 2008 simulation indicates that the Earth's orbit will decay due to tidal effects and drag, causing it to enter the red giant Sun's atmosphere and be vaporized.[213] After that, the Sun's core will collapse into a white dwarf, as its outer layers are ejected into space as a planetary nebula. The matter that once made up the Earth will be released into interstellar space, where it may one day become incorporated into a new generation of planets and other celestial bodies.

Moon

Characteristics
Diameter 3,474.8 km
Mass 7.349×1022 kg
Semi-major axis 384,400 km
Orbital period 27 d 7 h 43.7 m
Details of the Earth–Moon system. Besides the radius of each object, the radius to the Earth–Moon barycenter is shown. Photos from NASA. Data from NASA. The Moon's axis is located by Cassini's third law.
Full moon as seen from Earth's northern hemisphere

The Moon is a relatively large, terrestrial, planet-like satellite, with a diameter about one-quarter of the Earth's. It is the largest moon in the Solar System relative to the size of its planet, although Charon is larger relative to the dwarf planet Pluto. The natural satellites orbiting other planets are called "moons" after Earth's Moon.

The gravitational attraction between the Earth and Moon causes tides on Earth. The same effect on the Moon has led to its tidal locking: its rotation period is the same as the time it takes to orbit the Earth. As a result, it always presents the same face to the planet. As the Moon orbits Earth, different parts of its face are illuminated by the Sun, leading to the lunar phases; the dark part of the face is separated from the light part by the solar terminator.

Due to their tidal interaction, the Moon recedes from Earth at the rate of approximately 38 mm a year. Over millions of years, these tiny modifications—and the lengthening of Earth's day by about 23 µs a year—add up to significant changes.[214] During the Devonian period, for example, (approximately 410 mya) there were 400 days in a year, with each day lasting 21.8 hours.[215]

The Moon may have dramatically affected the development of life by moderating the planet's climate. Paleontological evidence and computer simulations show that Earth's axial tilt is stabilized by tidal interactions with the Moon.[216] Some theorists believe that without this stabilization against the torques applied by the Sun and planets to the Earth's equatorial bulge, the rotational axis might be chaotically unstable, exhibiting chaotic changes over millions of years, as appears to be the case for Mars.[217]

Viewed from Earth, the Moon is just far enough away to have almost the same apparent-sized disk as the Sun. The angular size (or solid angle) of these two bodies match because, although the Sun's diameter is about 400 times as large as the Moon's, it is also 400 times more distant.[133] This allows total and annular solar eclipses to occur on Earth.

The most widely accepted theory of the Moon's origin, the giant impact theory, states that it formed from the collision of a Mars-size protoplanet called Theia with the early Earth. This hypothesis explains (among other things) the Moon's relative lack of iron and volatile elements, and the fact that its composition is nearly identical to that of the Earth's crust.[218]
 
Scale representation of the relative sizes of, and average distance between, Earth and the Moon

Asteroids and artificial satellites

The International Space Station is an artificial satellite that orbits Earth.

Earth has at least five co-orbital asteroids, including 3753 Cruithne and 2002 AA29.[219][220] A trojan asteroid companion, 2010 TK7, is librating around the leading Lagrange triangular point, L4, of Earth in Earth's orbit around the Sun.[221][222]

As of 2011, there are 931 operational, man-made satellites orbiting the Earth.[223] There are also inoperative satellites and over 300,000 pieces of space debris. Earth's largest artificial satellite is the International Space Station.

Rejuvenation

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